''What we found is the first example of a hit man in your neurons; your neurons have a hit man in certain ones,'' said Dr. Dale Bredesen, a neurobiologist at the University of California at Los Angeles.

''This is an unusual phenomenon, and we're excited about it because it explains a lot of things that we didn't understand before,'' he said. ''The neurons that are affected earliest and most severely in Alzheimer's appear to be those that contribute to the confusion and forgetfulness.''

Bredesen, working with a team at UCLA and the Stanford Research Institute, writes in today's edition of the journal Science that neurons in the cerebral cortex, where memory and learning occur, tend to be those most likely to commit suicide.

Curiously, an identical population of cells in the brain stem, which controls respiration and heartbeat, do not kill themselves. Bredesen said it is not yet fully understood why such selectivity occurs, but it is likely that the suicide molecule does not reside in those cells.

Researchers working on the project say the suicide molecule first becomes active long before birth, during life's embryonic stage. It is needed by a vast range of tissues during the embryonic period, as a signal for the cells to decelerate growth before birth.

In the adult brain, these molecules apparently switch on again - and kill the cell - when they detect the absence of a vital neurochemical called nerve growth factor - or NGF.

The amount of NGF in the brain declines with age; in Alzheimer's patients it virtually disappears.

The molecule strikes primarily in neurons that produce the chemical acetylcholine. Without acetylcholine, memory and learning are lost, which may explain the early symptoms of Alzheimer's.

The experimental drug tacrine, approved by the federal government earlier this year for treatment of mild to moderate Alzheimer's symptoms, is aimed at preserving those acetylcholine-producing cells.

Bredesen believes the cell-suicide studies bolster genetic work such as that under way at the University of South Florida in Tampa, where scientists are seeking genes associated with the disorder.

Last year Tampa researchers led by Drs. John Hardy and Mike Mullan pinpointed a gene linked to the type of Alzheimer's that begins before the age of 55.

Although the suicide theory provides tantalizing new evidence about Alzheimer's, it does not fully explain the disease's cause. Scientists worldwide have been racing to find the mechanisms that trigger the condition, which eventually could lead to an effective treatment.

Alzheimer's can last 20 or more years before the victim dies, and usually affects people over the age of 65. The Alzheimer's Disease and Related Disorders Association, based in Chicago, estimates that 4 million Americans have the disease.

Varying estimates have put the number of Floridians with Alzheimer's at 300,000 to 400,000.